In the field of mining and engineering construction, rock drilling tools are essential tools needed to accomplish a wide range of challenging tasks involving drilling, excavation, and various other operations. However, in the event of certain deficiencies during the operation, it can potentially affect the quality and safety of the project. These deficiencies could be attributed to inadequate heating temperatures, prolonged holding periods, or irregular heating patterns.
The following defects may occur during the heating process of rock drilling tools:
Overheating and overburning
In the production process of rock drilling tools, excessive heating temperatures can lead to rapid growth of the steel’s crystal grains, resulting in a coarsened grain structure. This heating defect is called overheating. Identify whether the steel grain coarsens, often using the standard organization of the grain and coarse grain organization comparison method.
This comparison reveals that in the same unit area, the standard structure typically contains nearly a hundred grains, while the coarsened grain structure only has a few grains. Generally, finer grains in steel correspond to higher strength, whereas coarser grains indicate lower strength. Consequently, if the steel grains become coarsened due to improper heating practices during the production of rock drilling tools, the intensity of the heated part significantly diminishes, leading to premature breakage and a reduced lifespan of rock drilling tools.
If the heating temperature surpasses the steel’s critical overburning temperature, the grain boundaries may melt or partially melt, severely damaging the connections between grains. This heating flaw is known as overburning. When a rock drilling tool undergoes overburning during heating, the connection force between steel grains becomes exceedingly weak. If it falls on the ground, it will shatter into numerous pieces, rendering it unsuitable for further processing. Therefore, this kind of heating defect will be irreparable, and the only way is to cut off the burned area and reheat it. In cases of slight localized overheating, attempting to fashion it into a rock drilling tool is possible, and the grains affected by overheating will be very coarse, resulting in minimal strength. This kind of heating defect predominantly occurs in the drill rod heating process when visual detection is not possible, such as when the drill rod is buried in a medium like coke or bituminous coal. It’s only upon the removal of the drill rod from the heating medium that one can detect the occurrence of overburning by observing bright sparks emanating from the heated section. If a substantial number of sparks appear, a simple tap on the heating area will cause it to break.
In the context of induction heating, overburning is a common issue. It is often due to the exceptionally rapid heating speed, long time, or visual temperature is not allowed, all of which can lead to overburning defects.
During the heating process of the rock drilling tool, intense oxidation will occur on the surface, forming a layer of complex iron and oxygen compounds called iron oxide scale. This layer of iron oxide scale is both rigid and brittle. During forging, it will be depressed on the surface of the rock drilling tool, forming pits or pockmarks, which will cause the surface condition of the rock drilling tool to deteriorate and affect the drilling life of rock drilling tools. If the iron oxide scale falls off in the forging die chamber, it will accelerate the wear of the die and damage the die surface. It also makes the surface of rock drilling tools not smooth and affects the surface quality. Therefore, although oxidation of rock drilling tools during the heating process is inevitable, how to reduce the degree of oxidation is very important. The factors that affect the formation of the iron oxide scale are mainly heating temperature, heating time, and chemical composition of steel.
When steel is heated, the carbon content within its surface layer diminishes due to a chemical reaction with oxygen in the surrounding air. This phenomenon is called surface decarburization, or decarburization for short. Decarburization is also a heating defect.
The harm of decarburization to rock drilling tools is serious. Due to decarburization, the surface hardness of rock drilling tools significantly decreases and becomes rough, and the fatigue resistance strength also declines significantly. As per available information, the fatigue strength of decarburized steel is merely about one-fourth of that found in untreated, smooth steel. Decarburization will greatly reduce the service life of rock drilling tools.
In summary, numerous issues can arise during the production and usage of rock drilling tools. It is crucial to be vigilant about potential defects that may appear during the heating process and implement appropriate measures to prevent and rectify them. By doing so, we can effectively prolong the service life of rock drilling tools and enhance project safety.